Field of the Invention
The present invention relates to an optical system illuminating a surface to be illuminated, an exposure apparatus, an imprint apparatus, a method for manufacturing an article, an optical element, and a method for manufacturing an optical system.
Description of the Related Art
In the lithography process of the manufacturing of a semiconductor device, a liquid crystal display apparatus, or the like, an exposure apparatus is used that illuminates a mask (reticle) using an illumination optical system, and projects an image of a pattern of the mask through a projection optical system onto a substrate on which a photosensitive resist layer is formed.
In optical systems of an exposure apparatus, aspherical lenses for correcting various aberrations are used. Although an aspherical surface has a complicated shape, it requires high shape accuracy. So, in aspherical surface processing, a technique is used in which a minute polishing pad is scanned in a predetermined scanning direction while being in contact with the lens surface to polish the surface (Japanese Patent Laid-Open No. 2000-263408).
However, if a polishing pad is scanned in a predetermined scanning direction to polish the surface of an optical element as in Japanese Patent Laid-Open No. 2000-263408, linear polishing marks are left on the surface of the optical element along the scanning direction. Light is not easily transmitted through the polishing mark parts. Therefore, if a surface to be illuminated (substrate) is illuminated using an optical element having polishing marks, shadows such that light from the light source is blocked by the polishing marks are projected onto the surface to be illuminated. If, in an optical system of an exposure apparatus, significantly uneven illuminance is caused by shadows cast on the surface to be illuminated, a line width abnormality of the exposure pattern projected onto the substrate can occur. If, for example, linear polishing marks are about 1 mm in width, and uneven illuminance of 0.05% or more is caused by them, a line width abnormality of the exposure pattern can occur.